The present invention relates to methods for inhibiting corrosion of corrodible metals present in contact with water in distributive water systems, particularly those associated with distribution of drinking water.
In 1992, the United States Environmental Protection Agency adopted a standard for lead and copper in municipal drinking water. The standard sets action levels for lead and copper in standing samples collected from faucets with the highest risk for elevated lead and copper levels. The action level for lead is 15 xcexcg/liter of water; the action level for copper is 1.3 mg/liter of water. The source of these and other metals in drinking water is primarily corrosion of plumbing system components, which include copper and lead-based solder as well as carbon steel and brass. Current anticorrosion additives, which include phosphate salts and/or zinc salts, have been in use for many years, but do not always provide adequate protection. There remains a need for improved corrosion inhibitors which are not themselves likely to present a health hazard when added to water.
Co-owned U.S. Pat. No. 5,202,028 and No. 5,510,057 describe the use of stannous salts, such as stannous octanoate or stannous chloride, typically in combination with other additives, in an alcoholic solvent medium, for reducing metal corrosion in, for example, cooling water tower systems. Additional solubilizing agents were also generally used. However, these additional components, including the alcoholic solvent, would be undesirable or even prohibited for use in drinking water supplies. Alcoholic solvents can provide a food source for the growth of bacteria and thus impact the sterility of drinking water supplies. Reduction in microbial growth would be beneficial in industrial water supplies as well. Accordingly, there is a need for effective corrosion inhibitors which minimize organic content.
The present invention includes, in one aspect, a method for inhibiting corrosion of corrodible metal in contact with water in a distributive water system. According to the method, a composition consisting essentially of a stannous (Sn+2) salt of a non-carbon acid, or an aqueous solution thereof, is added to the water, and a concentration of the stannous salt corresponding to a tin level of about 0.01 to about 75 ppm, preferably about 0.05 to 25 ppm, is maintained in the water system.
The salt is preferably selected from stannous bromide, chloride, sulfate, nitrate, oxide, pyrophosphate, perchlorate, tetrafluoroborate, monofluorophosphate, ammonium fluoride, sodium fluoride, and fluorosilicate. Preferred salts include stannous bromide, chloride, sulfate, nitrate, and oxide. The stannous salt is preferably added in the form of an aqueous solution.
The corrodible metal is typically a ferrous metal, a brass metal, a copper-containing metal, or a lead-containing metal. In a preferred embodiment of the method, the distributive water system is a municipal drinking water system. Such a system may initially contain an alkali metal phosphate, such as a pyrophosphate, an orthophosphate, a hexametaphosphate, a hypophosphate, a polyphosphate, or a combination thereof, typically in an amount effective to provide a concentration of about 0.01 ppm to about 5 ppm of the alkali metal phosphate in the water system. The water system may also be treated to contain at least one component selected from a dispersing agent, a chelating agent, and a biocide.
In another aspect, the invention provides a related method for inhibiting corrosion of corrodible metal in contact with water in a distributive water system. In this method, a composition consisting essentially of a stannous (Sn+2) salt of a non-carbon acid, or an aqueous solution thereof, is added to the water system, in combination with an alkali metal phosphate selected from a pyrophosphate, an orthophosphate, a hexametaphosphate, a hypophosphate, and a polyphosphate. A concentration of the stannous halide corresponding to a tin level of about 0.01 to about 75 ppm is maintained in the water system. Again, the salt is preferably selected from stannous bromide, chloride, sulfate, nitrate, oxide, pyrophosphate, perchlorate, tetrafluoroborate, monofluorophosphate, ammonium fluoride, sodium fluoride, and fluorosilicate, and more preferably selected from stannous bromide, chloride, sulfate, nitrate, and oxide. The alkali metal phosphate, e.g. sodium hexametaphosphate or sodium orthophosphate, is typically present in an amount effective to provide a concentration of about 0.01 ppm to about 5 ppm in the water system.
In a further aspect, the invention provides a metal corrosion inhibiting composition, consisting essentially of (i) a stannous salt of a non-carbon acid, or an aqueous solution thereof, and (ii) an alkali metal phosphate selected from a pyrophosphate, an orthophosphate, a hexametaphosphate, a hypophosphate, and a polyphosphate. In preferred embodiments, the stannous salt is selected from stannous bromide, chloride, sulfate, nitrate, oxide, pyrophosphate, perchlorate, tetrafluoroborate, monofluorophosphate, ammonium fluoride, sodium fluoride, and fluorosilicate, and more preferably from stannous bromide, chloride, sulfate, nitrate, and oxide. The stannous salt and alkali metal phosphate are present in relative amounts effective to produce concentrations of about 0.01 to about 75 ppm tin and about 0.01 ppm to about 5 ppm alkali metal phosphate, respectively, in a distributive water system.
The composition may also include one or more substances typically added to a distributive water stream for purposes other than inhibition of corrosion, such as a dispersing agent, chelating agent, or biocide. In one embodiment, the composition includes an acrylate copolymer, e.g. an acrylate/sulfonate copolymer.
These and other objects and features of the invention will become more fully apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.